1. Field of the Invention
The present invention relates to the field of liquid crystal displaying, and in particular to a light guide plate with coating layer and a backlight module containing the light guide plate.
2. The Related Arts
Liquid crystal display (LCD) has a variety of advantages, such as compact device size, low power consumption, and being free of radiation, and is thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal panel and a backlight module. The working principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and a plurality of vertical and horizontal fine electrical wires is arranged between the two glass substrates, whereby the liquid crystal molecules are controlled to change direction by application of electricity in order to refract light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided by the backlight module in order to normally display images. Thus, the backlight module is one of the key components of an LCD. The backlight module can be classified as two types, namely side-edge backlight module and direct backlight module, according to the position where light gets incident. The direct backlight module arranges a light source, such as a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED) at the back side of the liquid crystal panel to form a planar light source that directly provides lighting to the liquid crystal panel. The side-edge backlight module arranges a backlight source, such as an LED light bar based light source, at an edge of a back panel that is located rearward of one side of the liquid crystal panel. The LED light bar emits light that enters a light guide plate through a light incident face of the light guide plate and is projected out through a light exit face after being reflected and diffused to thereby form, after transmitting through a set of optic films, a planar light source to be provided to the liquid crystal panel.
Referring to FIG. 1, the technical progress of using an LED light bar 100 as a light source makes a slim-bezel backlight module a promising trend of future development. To realize a design of slim bezel while ensuring reliability of an optic film assembly 200, the margin of the optic film assembly 200 that is depressed down and retained by a mold frame 300 is often of a very limited width so that it is often that the optic film assembly 200 is made extending to an edge of a light guide plate 400 in order to prevent the optic film assembly 200 from sliding off the backlight module. In this arrangement, the light incident end of the light guide plate 400 allows light from the LED light bar 100 to directly enter the optic film assembly 200 to be then reflected and directly projected outward.
To handle such a problem, a commonly used arrangement is to provide a shielding wall 302′ on a mold frame 300′ (see FIG. 2), in order to block light from directly entering the optic film assembly 200.
Yet, as shown in FIG. 3, due to the function of depressing and retaining effected by the mould frame 300′, the optic film assembly 200 is often stuck to the light guide plate 400, causing change of total internal reflection and thus allowing light to directly pass through the light guide plate 400 to enter optic film assembly 200 (propagation direction of light being indicated by arrows) so as to form bright lines along the light incident end of the light guide plate 400 and lead to loss of light.
Researches have shown that light leakage resulting from the optic film assembly being stuck to the light guide plate is because the light guide plate often has an index of refraction that is close to the optic films. Taking a light guide plate made of PMMA (poly methyl methacrylate) and an optic film made of PET (poly ethylene terephthalate) as an example, PMMA has a refractive index of 1.49, while PET has a refractive index that is greater than 1.50. When light travels in the light guide plate toward the contact interface, the light is moving from an optically thin medium into an optically thick medium. Thus, the light does not undergo total internal reflection and directly transmits into the optic films, thereby leading to light leakage.
Further, as shown in FIG. 4, to prevent light from emitting from a side face of the light guide plate 400, a non-incidence side face of the light guide plate 400 is often attached with a side reflection plate 500, which functions to allow light that emits out of the side face of the light guide plate 400 to be reflected back into the light guide plate by the side reflection plate 500 for increasing utilization efficiency of light. However, since the interface where the side reflection plate 500 is attached to the light guide plate 400 is generally not a smooth surface, scattering reflection of light often occurs at the interface, thereby resulting in light leakage.